Waifu is fan slang for a fictional female character a fan holds in special affection, jokingly framed as a spouse. It derives from the English "wife" as pronounced in Japanese, and its use is usually playful or ironic.
Waifu entered English-language fandom in the 2000s, from an anime line that turned the word wife into Engrish slang, and it settled into a playful institution. Declaring a waifu is a fandom ritual: half joke, half genuine statement of which character's story and design a fan finds most compelling, and lists, polls, and seasonal waifu contests are organized around the declaration.
The term's tone is the thing to get right. It is slang, usually ironic, occasionally sincere, and always about fictional characters; the fandom itself polices the boundary between enjoying the ritual and mistaking it for a relationship. Companion vocabulary includes husbando for male characters and a small zoo of derived jokes.
On this site the word appears only here, defined. Quizzes test what characters did in their stories, and a character is never presented to readers through this label. The glossary's job is to make the fandom's vocabulary legible, including the words the site itself does not use. Seasonal charts ranking characters by declared waifu status are a fixture of fan sites, running with a seriousness that is itself part of the joke everyone is in on.
In use"Asked for his waifu, a fan names the strategist whose arc he has defended in arguments for six years, entirely as the joke intends."